1. Field of the Invention
The present invention relates to a sputtering target made of a sintered body mainly composed of zinc sulfide and silicon dioxide for use in the formation of a protective layer for an optical recording medium capable of recording, reproducing and erasing data using a light beam, and a process for the preparation thereof.
2. Description of the Related Art As a protective layer for optical disc capable of recording, reproducing and erasing data using light beam, there is used a layer made of a mixture of zinc sulfide (ZnS) and silicon dioxide (SiO.sub.2). Further, as a process for the formation of such a protective layer, there is used sputtering process using a sintered body made of zinc sulfide and silicon dioxide as a sputtering target.
The production of the sintered body made of zinc sulfide and silicon dioxide to be used as a sputtering target for use in the formation of a protective layer for an optical recording medium such as optical disc is carried out by a CIP (Cold Isostatic Pressing) process and a HIP (Hot Isostatic Pressing) process in combination, hot pressing process or the like. Among these processes, the hot pressing process is mainly employed on an industrial basis.
However, sintered ceramics made of zinc sulfide and silicon dioxide are liable to crack during production. This defect is known as a main cause that lower the yield. It has been of urgent necessity to prevent cracking occurring during production.
Zinc sulfide and silicon dioxide, which are constituents of a sintered body to be used as a sputtering target for use in the formation of a protective layer for optical recording medium, undergo little solid solution in each other even if sintered under pressure. Further, zinc sulfide and silicon dioxide exhibit different thermal expansion coefficients and thus cause the occurrence of residual stress on the interface of crystalline particles in the sintered body when the sintered body is allowed to cool from the sintering temperature to room temperature during the production of sintered body by sintering. If the sintered body adsorbs water from air with this stress remaining, it shows a volume increase and hence can easily crack.
In particular, in order to obtain a sintered body having a higher density, it is advantageous to sinter the material at high temperatures. However, if the material is sintered at a temperature as high as not lower than the transition temperature of zinc sulfide from zincblende structure to wurtzite structure, the foregoing effect becomes more remarkable, thereby making it difficult to obtain a sintered body free of cracking.
Further, if the sputtering target exhibits a low density, it has a low thermal conductivity and flexural strength and an insufficient mechanical strength and thus cracks during sputtering. Accordingly, a high sputtering power cannot be applied. As a result, the throughput of optical disc cannot be raised. Moreover, such a low density sputtering target has a large number of voids in its structure. These voids form an aggregate of redeposits called "nodule" that gives a source from which particles are produced. The particles can lower the yield of products.